1. Field of the Invention
The present invention relates to a laminated duplexer applicable to mobile communication terminals such as mobile phones, and more particularly to a matching circuit for performing matching of characteristic impedance between an antenna terminal and each of transmitting and receiving terminals, and isolation between transmitting and receiving frequencies, which matching circuit is configured to reduce the physical length of its conductor pattern, thereby being capable of achieving an improved miniaturization thereof, a reduction in insertion loss, an improvement in the reflection characteristics of an associated antenna, and, thus, an improvement in bandpass characteristics, and a laminated duplexer with the matching circuit.
2. Description of the Related Art
Generally, integrated duplexers of a bulk type have a drawback in that it is difficult to reduce the size thereof, even though they are superior in terms of performance. Although SAW duplexers can achieve miniaturization, there are drawbacks in that they have a low power capacity and a high sensitivity to humidity and temperature while being relatively expensive, as compared to the bulk type integrated duplexers. On the other hand, laminated duplexers can achieve miniaturization while being sufficiently competitive in terms of the manufacturing costs. The laminated duplexers are superior over the SAW duplexers in terms of power capacity, while having a high resistance to humidity and temperature. Of course, the laminated duplexers exhibit an inferior performance to the bulk type integrated duplexers or SAW duplexers. For this reason, active research for improving the performance of such laminated duplexers is being conducted.
If good results are obtained from the research for improving the performance of laminated duplexers, it may then be expected that the laminated duplexers will replace the bulk type integrated duplexers or SAW duplexers.
In order to achieve an improvement in the performance of such laminated duplexers, it is necessary to mainly conduct research with respect to the following factors:                (1) Material: Low temperature cofired ceramic (LTCC) of an intermediate dielectric constant (relative dielectric constant≈20˜100) having a high Q value (>1,500);        (2) Electrode: Electrode material having a high conductivity (>4.83×107 simens/m);        (3) Resonator Structure: Resonator structure having a Qu value; and        (4) Matching Circuit: Matching circuit has to completely isolate transmitting and receiving filters from each other while minimizing a possible degradation in the transmitting and receiving filters.        
FIG. 1 is a block diagram illustrating the basic configuration of a general duplexer. As shown in FIG. 1, such a duplexer mainly includes a transmitting filter, a receiving filter, and a matching circuit for coupling the filters. The matching circuit serves to minimize interference between the transmitting and receiving filters caused by the coupling of those filters. Accordingly, the matching circuit should be designed to minimize the influence thereof on the electrical characteristics of the transmitting and receiving filters, for example, insertion loss.
An example of conventional laminated duplexers is disclosed in Japanese Patent Laid-open Publication No. 2002-164710. The disclosed laminated duplexer will now be described with reference to FIGS. 2 to 4.
FIG. 2 is a perspective view illustrating the conventional laminated duplexer represented by the reference character A. Referring to FIG. 2, “1” represents a dielectric (laminate), “2a” and “2b” ground electrodes, “3” strip lines, that is, strip lines 30 to 35, “4” an inner wiring terminal, “5” a transmitting filter, “6” a receiving terminal, and “7” a matching circuit.
The laminate 1 consists of a plurality of laminated dielectric layers 1a. For the material of the laminate 1, a mixture of a dielectric ceramic material and a low temperature firing oxide or a low melting point glass material may be used. The dielectric ceramic material may include BaO—TiO2-based ceramic, Ca—TiO2-based ceramic, MgO—TiO2-based ceramic, etc. The low temperature firing oxide may include BiVO4, CuO, Li2O, B2O3, etc. For miniaturization of the matching circuit and filters, it is necessary to use a high dielectric constant material having a relative dielectric constant of, for example, 15 to 25. Each dielectric layer 1a has a thickness of about 50 to 3,000 μm.
The ground electrodes 2a are formed at upper and lower surfaces of the laminate 1, respectively, whereas the ground electrodes 2b are formed at side surfaces of the laminate 1, respectively. Each ground electrode 2a or 2b is made of a conductor material containing, as a major component thereof, Ag and Cu (Ag group, Ag alloy such as Ag—Pd or Ag—Pt, Cu monomer, or Cu alloy).
FIG. 3 is an enlarged view illustrating a part of the matching circuit shown in FIG. 2. FIG. 4 is an equivalent circuit diagram of the receiving filter and matching circuit shown in FIG. 2.
Referring to FIGS. 3 and 4, the matching circuit 7 has a T-shaped circuit structure including a capacitor C2 formed between capacitor electrodes 4b and 4c connected to an antenna terminal 42 of the receiving filter 6 in series, a capacitor C0 formed between an edge-side strip line of the receiving filter 6, that is, the strip line 32, and a capacitor electrode 4d facing the strip line 32, and an inductor L1 formed of a coil 400. In the matching circuit 7 having such a configuration, the impedance characteristics of the receiving filter 6 are adjusted in accordance with the phase characteristics of a capacitor Ci formed between the capacitor electrode 4d and a main strip line portion 32a of the strip line 32, in order to achieve desired matching. The coil 400 includes bent electrodes 41a to 41c, and via holes 42a to 42c. 
Since the matching circuit 7 of the above mentioned conventional laminated duplexer has a coil formed to have a spiral shape in the dielectric, using a plurality of bent electrodes and via holes, it can achieve miniaturization.
That is, where the matching circuit of the conventional laminated duplexer has a spiral coil, as mentioned above, it is possible to reduce the coil size in a longitudinal direction. However, the coil increases in size in a thickness direction correspondingly to the reduction in the longitudinal size, so as to provide a desired electrical length required in the matching circuit, even though the increase in thickness may vary more or less in accordance with a variation in the spiral shape of the coil. For this reason, there is a limitation on the miniaturization in both the longitudinal direction and the thickness direction.
Thus, only a limited miniaturization is achieved where the coil of the matching circuit is simply formed to have a spiral shape or formed using bent electrodes in order to miniaturize the duplexer applicable to a mobile communication terminal such as a mobile phone while maintaining the electrical length required in the matching circuit. Accordingly, it is necessary to research and develop a new laminated duplexer capable of overcoming the limitation.